1 January 2009

Until recently, it was generally believed that high GI foods cause blood glucose levels to rise rapidly to relatively high levels, before dropping back to fasting levels, in an equally short period of time, providing a quick ‘spike’ of energy. Low GI foods, on the other hand, were thought to cause blood glucose levels to rise gradually over a relatively longer period of time, reaching a modest peak, before gently returning to baseline over a longer period.

The ‘Healthy Curves’ story in this issue of GI News not only suggests that the normal response depends on what carbohydrate food has been consumed, but that the notion that a low GI food has a uniquely long tail or extended glucose profile is not correct. Although a slowly digested starch or sugar may represent a slow-release form of energy, this does not automatically imply that a low GI food produces a sustained glucose response, as their metabolic energy can be released through alternative energy pathways.

What Brand-Miller’s study shows is that blood glucose levels tend to return to the baseline sooner after consuming sugary foods such as soft drinks and juices, regardless of their GI. These findings can be explained by the fact that commonly consumed sugars such as sucrose, lactose and fructose – regardless of their source – contribute fewer glucose molecules than the same weight of starch. Fifty grams of sucrose, for example, contains only 25 g of glucose versus 50 g of glucose in starch.

Does that mean you can still obtain sustained energy from lower GI sugars?We have to get a bit technical here, so please forgive us. While lower GI sugars may have a negligible affect on blood glucose levels, the 25 g of fructose or galactose from a 50 g serve of sucrose or lactose still provide the body’s cells, tissues and organs with energy it can metabolise. After reaching the liver, fructose is rapidly removed from the blood stream, phosphorylated, and enters the glycolytic pathway, usually ending up as pyruvate and adenosine triphosphate (ATP), our body’s main energy ‘currency’. Similarly, galactose is extracted from the blood and converted to glucose in the liver, and again converted to pyruvate and ATP, just like dietary glucose. Under normal aerobic conditions, pyruvate is converted via the citric acid cycle to ATP, producing more energy. Therefore, ingestion of low GI sugars such as sucrose or lactose may have a negligible affect on blood glucose levels, but this does not mean they do not still help provide our body’s cells, tissue and organs with a sustainable source of energy. And there is considerable evidence linking the consumption of meals with a low GI to sustained physical and improved mental performance in humans. Please contact me for the references to these studies if you wish. Email: alan@gisymbol.com